This invention relates to a method and apparatus for removing excess zinc from an article coated with molten zinc. In particular, it relates to a method and apparatus for spinning articles dipped in molten zinc to remove the excess zinc.
In conventional galvanizing methods, the metal article to be coated undergoes five distinct treatments. Initially, the metal article is passed through a caustic bath where oils and greases are eliminated and the metal is thoroughly washed clean. A typical caustic used in this first stage bath is sodium hydroxide.
From the first stage caustic bath, the cleaned metal article is passed into a second stage water-rinse bath where caustic from the first bath is removed. The third stage bath in the process contains an acid such as hydrochloric acid. This third stage bath is termed the "pickling" bath. It is in this third stage bath that the surface preparation of the metal article to be galvaninzed occurs. Thus, the pickling bath removes iron oxide from the metal object to be galvanized.
From the pickling bath, the metal article passes to a fourth stage bath, termed the "preflux" bath, which conventionally contains a solution of ammonium chloride and zinc chloride. It is in this preflux bath that the metal article undergoes secondary surface preparation, wherein wetting agents, such as ammonium chloride and zinc ammonium chloride, are added to the surface of the metal object.
The fifth and final treatment bath in the galvanizing process is a tank commonly containing a molten metal such as zinc. The article is submerged in molten metal to apply a coating to the article prepared as previously described in the initial four treatment stages.
Conventional methods of galvanizing articles usually include a spinning step to throw off excess molten metal or "spelter". After the coating step previously described, the articles are removed and usually placed in a floor mounted centrifuge and spun to remove the spelter. This spelter strikes the sides of the centrifuge and is collected for subsequent removal and reuse. The articles are then removed from the centrifuge while they are still hot and submerged in a quench tank. The thusly coated articles are then removed from the quench tank for sorting and packaging.
There are many drawbacks to the above-described conventional galvanizing process, but it has been in use for many years with only minor modifications. A major drawback of the conventional floor mounted centrifuge process has been that the articles cannot be accelerated quickly to rotational speed due to the relatively low torque of the conventional electric or air motors employed in conventional floor mounted centrifuges. Further, even if the centrifuge was capable of greater torque and therefore faster rotational acceleration, the great inertia of the basket and articles imparts such high resistance to acceleration that the floor of the centrifuge slips relative to the basket containing the articles coated with the zinc so that rapid acceleration or deceleration cannot be achieved. To minimize this problem, the weight capacity of the centrifuge is kept low, conventionally about 100 to 200 pounds.
An alternative apparatus for spinning articles in a galvanizing process is the suspended centrifuge. An example of the suspended centrifuge technique is described in U.S. Pat. No. 3,699,918 to Garrison. In this technique, a perforate basket containing the work to be galvanized is suspended from a air motor which, in turn, is suspended from a hoist. This motor and hoist assembly travels along an overhead track which permits the basket suspended from the assembly to be dipped in each of the pretreatment baths, as desired. After pretreating, the basket is placed in the zinc kettle to be cooked in the conventional manner. Thereafter, the hoist and motor assembly lifts the basket from the zinc bath and spins the basket over the zinc bath permitting the spelter to fall back into the kettle. After spinning, the hoist and motor assembly places the basket in the quench tank or places the basket in an unloading station wherein the contents of the perforate basket is dumped into the quench tank.
Although the suspended centrifuge techniques provide larger load capacities and the parts are spun more quickly after removal of the basket from the kettle than with the conventional floor mounted centrifuge, it too has several disadvantages. In the suspended centrifuge method, the basket is coupled to the hoist and motor assembly by a hook through an eye on the basket. By necessity, the hook must be small enough to be quickly and easily passed into the basket eye, however the hook must be strong enough to withstand repeated transmission of rotational torque to the basket. These competing interests have resulted in a hook which is prone to breaking under stress.
It is apparent that breaking stress is most likely to occur while the basket is spinning over a tank of molten metal. If the hook breaks, a basket weighing several hundred pounds and spinning approximately 500 rpm is dropped from a height into a kettle of molten metal exposing the workers nearby to serious injury.
A second serious drawback is the limitation of acceleration torque which can be applied to the basket due to the inertia of the basket in relation to the mass of the free swinging motor and hoist assembly. When the basket is suspended and the motor is accuated, the tendency is for the motor and hoist assembly to turn and the basket to remain motionless. To overcome this tendency, a long metal handle is attached to the motor and hoist assembly to be held by an operator. Although the length of the handle permits a mechanical advantage, the acceleration and deceleration is limited by the strength of the operator. Further, given that the motor is started and stopped about every seven minutes, fatigue by the operator may result in the handle being pulled from the hand of the operator to strike another worker or to strike the operator upon completion of a revolution.
These and other disadvantages of the prior art are overcome by the present invention, however, and improved methods and apparatus are provided for removing excess molten metal from articles in the galvanizing process.